Penning type vacuum pumps



July 25, 1967 L. A. HQLLAND PENNING TYPE VACUUM PUMPS 4 Sheets-Sheet, l

Filed June 9, 1964 ImvaNw-oa Lasne A. HoLLAND.

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BV MI AroseNEv 3,332,606 PENNING TYPE VACUUM PUMPS Leslie Arthur Holland, Crawley, England, assignor to Edwards High Vacuum International Limited, Crawley, England, a British company Filed June 9, 1964, Ser. No. 373,799 3 Claims. (Cl. 230-69) This invention relates to Penning type vacuum pumps and more particularly to such pumps in which chemically active cathode materials for sorbing gas in vacuum are used. A known form of pump consists of a pair of parallel cathode electrodes with an anode, or multiple anodes, in the inter-space. A magnetic field is used the lines of force being normal to the cathode planes. As is well known the combination of the applied magnetic and electrical fields results in electrons leaving the cathode following spiral paths between the two cathodes before being captured. As is known to those skilled in the art during this spiraling electrons collide with atoms of gases within the chamber in and around the anode, and these collisions produce more free electrons and further result in the forming of positive ions. (Basis: Prior art knowledge recited in patent to Jepson of record col. 3, lines 21-25.) The spiral paths greatly lengthen the trajectories of the electrons giving a high yield of positive gas ions at low gas pressures. The positive ions accelerated by the electric eld strike the cathodes removing electrons and metal atoms. If the cathodes are made of chemically active materials, e.g. titanium or zirconium then the sputtered metal condenses on the vacuum envelope and anode electrode where it sorbs gases by various processes such as the formation of chemical compounds, chemisorption, and diffusion. Continuous sputtering produces fresh layers for adsorption and covers over existing gas containing layers.

As the sputtering process ensues an equlibrium state is reached by which the rate of entry of gas ions into the cathode is balanced by their rate of release with the removal of sputtered metal. However, at the edges of a bombarded cathode the current density of the positive ions is usually less than that at the centre of the cathode. Consequently the sputtering rate is lower in this region and there is a net transfer of sputtered metal on the cathode surfaces i.e. sputtered material can accumulate at the edges of the cathode. Inert gas molecules cannot easily be sorbed on any surfaces except that of the cathode to which they are directed as positive ions. In the central region of the cathode such positive ions cease to be pumped after an induction period, however they are trapped in the sputtered material at the fringes of the cathode. Operation of this type of Penning pump has shown that the pump can become unstable and cease to sorb inert gas ions in the sputtered material on the cathode periphery if the discharge wanders over the cathode surface heating redeposited cathode material and sputtering it from the surface. When such an effect occurs there is a rise of pressure in the system as trapped gas is released. The object of the present invention is to enhance the sorption of inert gas atoms and prevent pumping instability.

Accordingly the present invention contemplates a Penning type ion pump provided with an electrode assembly comprising a cellular anode disposed between a pair of parallel cathodes which are adapted and arranged to be maintained at the same potential, and means for impressing a magnetic field across said electrode assembly in a direction normal to the cathode planes to cause electrons moving in the space within the assembly and within open cells of said cathode to follow spiral paths between said cathodes before being captured, said pump particularly characterized in that said cellular anode comprises nited States Patent O 3,332,606 Patented July 25, 1967 Vice a first set of active cells of dimensions through which the spiraling electrons may pass to sustain a discharge, and a second set of inactive cells through which such spiraling electrons may not pass, said inactive cells having at least one wall in the path of the spiraling electrons to obstruct their passage for preventing a discharge.

In one construction of such a pump the said second set of inactive cells comprises open cells having crosssectional dimensions smaller than those of the active cells.

In a second suitable construction, the second set of inactive cells comprises cells that are closed and interspersed among the open cells, whereby said cells are inactive.

Alternative forms of electrode assembly embodying the invention will now be described in greater detail by way of example with reference to the accompanying drawings in which:

FIGURE 1 is a fragmentary explanatory diagram;

FIGURE 2 is a diagrammatic plan view of part of an electrode assembly;

FIGURE 3 is a diagrammatic side elevation of an assembly including electrodes as shown in FIGURE 2;

FIGURE 4 is a diagrammatic perspective view of the anode included in FIGURE 3;

FIGURE 5 is a plan view of an alternative form of anode;

FIGURE 6 is a side elevation of the anode shown in FIGURE 5;

FIGURE 7 is a further explanatory diagram;

FIGURE S is an elevation partly in section of a pump embodying the invention; and

FIGURES 9, l0 and l1 each show pumping seed ClllVeS.

Referring to the drawings, FIGURE l illustrates the effect produced when the conventional form of cellular anode is used. The broken lines 1 represent the projection on to the surface of a cathode 2 of the edges of the anode cells and the shaded areas 3 show the region where a net transfer of sputtered metal can occur.

In carrying out the present invention, the anode electrode is constructed to produce enlarged areas where such net transfer of sputtered metal can occur and FIGURES 2, 3 and 4 show an anode 4 in which the usual single thin walls are replaced by double walls 5. The anode is disposed between a pair of parallel cathodes 6 composed of titanium. The provision of the double walls 5 has the effect of producing spacing of the individual anode cells from each other by rectangular cells 7. The discharge conditions are such that a cold cathode discharge can be sustained within each of the square shaped cells but the dimensions of the rectangular cells are such that a discharge in this region is not possible. By this means each active discharge cell is separated from another active cell by a greater distance than can be obtained easily using a thick-walled anode. The areas on which sputtered material which is redeposited on the cathode surface can now be greatly increased in size and the material deposited in zones farther away from the active bombardment regions of the cathode. Consequently it becomes impossible for the intense discharge region to wander over the cathode surface and reach the most remote redeposited material. The principal sputtering zone of high current density is shown at 8 and the areas where a net transfer of sputtered material can occur are shown at 9.

Referring now to FIGURES 5 and 6, an anode with closed cells 10 and open cells 11 is shown and it will be seen that certain of the open cells are surrounded on four sides by enclosed cells which, from the discharge point of view, are inactive. Thus, such surrounded cells each has projected on the cathode an area of four times the discharge area in which it is possible for sputtered material to be redeposited on the cathode. Such enlarged areas are illustrated by the shaded portions 12 of FIGURE 7. The enclosed cells of the anode can be made by using solid pieces of metal slipped into the apertures of the boxes, this method being preferable to enclosing the boxes with lids because of difficulties in removing the entrapped gas during the pumping cycle.

In the application of the invention the double Wall anode construction described with reference to FIGURES 2, 3 and 4 presents advantages because each of the inactive rectangular cells or apertures 7, apart from inducing inactive discharge zones ot large area on the cathode, also has internally exposed surfaces for collecting sputtered material which can sorb chemically active gases.

Referring now to FIGURE 8, the pump shown consists of a body '13 supporting a magnet 14 and provided With a terminal 15 for the operating voltage supply leads for the electrodes. The anode 4 is of the general form shown in FIGURE 4 and the parallel cathodes 6 are of the general form shown in FIGURE 3. The individual anode cells are half an inch square and half an inch deep, the rectangular spacing cells 7 (FIGURE 4) being one eighth of an inch wide. The cathodes 6 are spaced one inch and a quarter apart. The magnet 13 provides a eld of 1000 gauss and the operating voltage is 6 kv.

The pumping speeds indicated by the curves shown in FIGURES 9 and 10 are in litres per second at specific pressures 1and the pressures in torr values. Thus, in the particular example of the pump being described, the pumping speed for air as shown by the curve 16 is of the order of 16 litres per second at a pressure of 10-5 torr measured at 10-V5 torr. The curves in FIGURE 10 show the pumping speeds obtained in the case of argon and curve 17 shows a pump speed of the order of 16 litres per second at a pressure of 10-5 torr, measured at 10-5 torr.

The volumetric speeds may be measured using two chambers one of which is constituted by the pump, the two chambers being separated by an oriced plate. The pumping speeds are determined by measuring the pressures in the two vessels, the conductance of the orifice being known.

FIGURE 11 shows a curve 18 which indicates the stability of argon pumping with time given in hours, the

saturation test being carried out at a pressure of 5 105 torr.

It will be understood that the invention may be carried out in ways different from the particular embodiment described. For example, the dimensions of the electrodes, their relative spacing and the operating voltages may be selected to suit required conditions.

I claim:

1. A Penning type ion pump of the sort provided with (A) an electrode assembly comprising (l) a pair of mutually spaced parallel cathodes which are adapted and arranged to be maintained at the same potential and (2) an anode having a plurality of open cells disposed between said cathodes, and` (B) means for impressing a magnetic held across said electrode assembly in a directional normal to the cathode planes to cause electrons moving in the spaces within said assembly and Within said open cells to follow spiral paths between the said cathodes before being captured, said pump particularly characterized in that (C) said cellular anode comprises l) a first set of active cells of dimensions through which the spiraling electrons may pass to sustain a discharge, and

(2) a second set of inactive cells through which such spiraling electrons may not pass, said inactive cells having at least one wall in the path of the spiraling electrons to obstruct their passage for preventing a discharge.

2. An ion pump as dened in claim 1, wherein said second set of inactive cells comprises open cells having smaller cross-sectional dimensions than those of the active cells.

3. An ion pump as defined in claim 1, wherein said sec- 0nd set of inactive cells comprising cells that are closed and interspersed among the open cells whereby said cells are inactive.

References Cited UNITED STATES PATENTS 3,161,802 12/1964 Jepsen etal 313-161 DAVID J. GALVIN, Primary Examiner. 

1. A PENNING TYPE ION PUMP OF THE SORT PROVIDED WITH (A) AN ELECTRODE ASSEMBLY COMPRISING (1) A PAIR OF MUTUALLY SPACED PARALLEL CATHODES WHICH ARE ADAPTED AND ARRANGED TO BE MAINTAINED AT THE SAME POTENTIAL AND (2) AN ANODE HAVING A PLURALITY OF OPEN CELLS DISPOSED BETWEEN SAID CATHODES, AND (B) MEANS FOR IMPRESSING A MAGNETIC FIELD ACROSS SAID ELECTRODE ASSEMBLY IN A DIRECTIONAL NORMAL TO THE CATHODE PLANES TO CAUSE ELECTRONS MOVING IN THE SPACES WITHIN SAID ASSEMBLY AND WITHIN SAID OPEN CELLS TO FOLLOW SPIRAL PATHS BETWEEN THE SAID CATHODES BEFORE BEING CAPTURED, SAID PUMP PARTICULARLY CHARACTERIZED IN THAT (C) SAID CELLULAR ANODE COMPRISES (1) A FIRST SET OF ACTIVE CELLS OF DIMENSIONS THROUGH WHICH THE SPIRALING ELECTRONS MAY PASS TO SUSTAIN A DISCHARGE, AND (2) A SECOND SET OF INACTIVE CELLS THROUGH WHICH SUCH SPIRALING ELECTRONS MAY NOT PASS, SAID INACTIVE CELLS HAVING AT LEAST ONE WALL IN THE PATH OF THE SPIRALING ELECTRONS TO OBSTRUCT THEIR PASSAGE FOR PREVENTING A DISCHARGE. 